As is well known, to improve the fuel efficiency or the driving performance of automobiles or to absorb the increase in weight accompanying safety measures or fuller options, reduction of the weight of automobiles is being sought. For this reason, for example, high strength steel plate is being used to reduce the thickness of the body structure and thereby lighten the weight of the body.
For example, when using 590 MPa class high strength steel plate to lighten the body, it is considered possible to secure the body strength while reducing the weight by about 40% compared with conventional steel plate. Very great results have been anticipated.
On the other hand, since an automobile receives force from bumps on the road surface while driving, impact when riding over road shoulders etc., and various other forces, torsional stiffness is required in addition to body strength. However, if using high strength steel plate to reduce the thickness of the body structure, even if the body strength is secured, the torsional stiffness generally falls.
That is, in high strength steel plate, the tensile strength of the steel plate is improved by the temperature history, ingredients, etc., but the Young's modulus of iron is constant and does not change. For this reason, if the body structure is reduced in thickness, the polar moment of inertia of area becomes smaller. As a result, the torsional stiffness falls.
Therefore, when using high strength steel plate etc. to maintain the body strength while reducing the thickness of the body so as to lighten the weight, it is also necessary to improve the torsional stiffness.
Regarding the improvement of the torsional stiffness of the body, as art focusing on the front body structure, for example, art such as shown in PLTs 1 to 4 is disclosed.
Specifically, PLT 1 discloses to form an upper part of a strut tower at one part of a wheel apron reinforcing member which is provided at an upper edge of a wheel apron panel which forms the wheel housing and connects the wheel apron reinforcing member to a subframe which extends in the engine compartment in the vehicle width direction.
PLT 2 discloses a front body structure provided with reinforcing frame members with front ends which are connected to rear parts of front side frames and with rear ends which are connected to side panels at rear parts of strut towers and a dash cross member which is connected to rear parts of the reinforcing frame member sand which extends at the bottom of a cowl part in the width direction.
PLT 3 discloses provision of a dash member at a dash panel which extends at the rear of the engine compartment in the width direction.
PLT 4 discloses to form strut housings as single parts and to join side members, hood ridges, a dash panel, and a cowl top panel to the strut housings to integrally form them together.